test: emacs: new tests "notmuch-show: {show, hide} message headers"

test: emacs: new tests "notmuch-show: {show, hide} message headers"

* test/emacs:

  - New subtest "notmuch-show: show message headers":
    Setting `notmuch-message-headers-visible' to t causes all headers
    defined in `notmuch-message-headers' to be shown.

  - New subtest "notmuch-show: hide message headers":
    Setting `notmuch-message-headers-visible' to nil causes all headers
    defined in `notmuch-message-headers' to be hidden.
    ("Subject:" may be an exception;  See the use of `headers-start' in
    `notmuch-show-insert-msg')

  - New subtest "notmuch-show: hide message headers (w/ notmuch-show-toggle-headers)":
    Setting `notmuch-message-headers-visible' to t causes all headers
    defined in `notmuch-message-headers' to be shown, but they can be
    hidden for the current message by running `notmuch-show-toggle-headers'.